The pun is that the "yu" sound makes you pucker your lips in the shape of a stereotypical kiss; she kisses him when his lips come together. He wanted her to pucker her lips the same way but she managed to say his name without doing it so he didn't kiss her.
Yu?Say "Mochizou", then!"Hot water"?
yu = hot watertee-hee!Say "yu"!!Why "yu"...?Mochizo!!smoochU...Mochizo.
in Japanese "ou" is pronunced as a long "o".MochizoU.Huh, well, whatever...
if there's a pun, I don't get it.